The present invention relates to a multi-beam optical scanner and more particularly to a multi-beam optical scanner realizing a light spot in an appropriate form on a scanned surface and effectively reducing degradation in image quality of a recorded image due to pitch deviation.
An optical scanner has been known in relation to an image forming apparatus, such as a digital copying machine, an optical printer, and an optical printing machine or the like. In the optical scanner as described above, there has been proposed a multi-beam optical scanning system for optically and concurrently scanning an image with a plurality of scanning lines for the purpose of speeding up an operation for writing images by way of optical scanning.
In the multi-beam optical scanning system, there is sometimes a case where scanning lines for optically and concurrently scanning are not adjacent to each other. There has been proposed, for instance, in Japanese Patent Publication No. HEI 6-418846  48846, a so-called xe2x80x9cinterlace scanningxe2x80x9d type of multi-beam optical scanner in which interlace scanning is executed by means of three scanning lines without one line therebetween.
In the interlace scanning as described above, selection of a signal for modulating each beam is irregular, so that optical scanning is easily complicated, and in addition, scanning lines for optically concurrently scanning are also largely spaced therebetween, so that xe2x80x9ca rate of pitch deviationxe2x80x9d of optical scanning due to a bend in the scanning lines becomes large, which makes it easy for image quality of a recorded image to degrade.
In the multi-beam optical scanner, there are strict restriction over a magnification in an image-formation system provided in a space between the light source and the surface for scanning due to a relation between pitches of scanning lines, and because of the restrictions over a magnification as described above, a position of an optical system to be provided in a side of the light source from the optical deflector must be closer to the optical deflector, and for this reason, a layout of optical arrangement becomes difficult.
Further, if a xe2x80x9chybridxe2x80x9d combination of two or more LD light emitting sections or LED light emitting sections is employed as a light source having a plurality of light emitting sections used in the multi-beam optical scanner, there occurs a problem of xe2x80x9cwavelength deviationxe2x80x9d that wavelengths of combined light emitting sections are not identical to each other, and when this wavelength deviation is present therein, constant velocity characteristics of optical scanning or the like may vary for each light emitting section.
What is known as a multi-beam optical scanning system is disclosed in Japanese Patent Publication No. HEI 7-111509, but in this optical scanning system a focal length of a lengthy lens for correcting surface offset is as short as 15 mm, and for this reason the lengthy lens is provided at a position close to the scanned surface, which makes larger a length in a direction corresponding to the main scanning as well as cost of the lengthy lens higher, and also toner splashed from a developing device generally provided adjacent to the scanned surface easily makes the lengthy lens contaminated.
It is an object of the present invention to effectively reduce degradation in image quality in a recorded image due to pitch deviation, in addition, to enable effective reduction of cost of a lengthy lens included in an optical system for converging deflected light fluxes on the scanned surface, and also to effectively suppress contamination of the lengthy lens due to splashed toner.
It is another object of the present invention to realize a multi-beam optical scanner in which a layout of optical arrangement is easy arid which is hardly affected by wavelength deviation in the light source.
The multi-beam optical scanner according to the present invention comprises a light source for a multi-beam, a coupling lens, a first image-formation system, an optical deflector, and a second image-formation system.
Herein, the light source for a multi-beam is a monolithic light source in which a plurality of LD light emitting sections or LED light emitting sections are arranged in a direction corresponding to an auxiliary scanning. The light source for a multi-beam can also comprise a plurality of LD light emitting sections or LED light emitting sections by means of hybrid combination thereof. A plurality of light emitting sections obtained by the hybrid combination thereof are different bodies from each other.
Herein the direction corresponding to auxiliary scanning is defined as a direction corresponding to a direction of auxiliary scanning in parallel on a virtual light path linearly extending along an optical axis in a light path from the light source to the scanned surface.
The coupling lens is a lens for coupling a plurality of light fluxes from the light source for a multi-beam to an image-forming optical system (optical system for forming on the scanned surface images in a plurality of light emitting sections in the light source for a multi-beam). A mode of coupling can be a mode for converting light fluxes from light emitting sections coupled to each other to parallel light fluxes or a mode in which each of light fluxes becomes one having weak converging performance or weak diverging performance.
The first image-formation system is an optical system for focusing a plurality of light fluxes coupled by the coupling lens and forming an image as a plurality of line images each long in a direction corresponding to the main scanning, and can use a convex cylinder lens or a concave cylinder mirror without having power in the direction corresponding to the main scanning. Wherein a direction corresponding to main scanning indicates a direction corresponding to a direction of main scanning in parallel on the virtual light path.
The optical deflector is a means having a deflecting reflection surface provided adjacent to a position for forming an image out of a plurality of line images for deflecting a plurality of light fluxes, and a known polygon mirror, a rotating double-face mirror, or a rotating single-face mirror or the like may be used for this purpose.
The second image-formation system is an optical system for separating a plurality of light fluxes deflected by the optical deflector from each other in an auxiliary scanning direction on a scanned surface and converging the plurality of light fluxes as a plurality of light spots optically scanning the surface to be scanned in accordance with deflection of the light fluxes, and includes a lengthy lens provided on the side of a scanned surface. Namely, the second image-formation system comprises a fxcex8 lens and a lengthy lens provided on the side of the scanned surface thereof.
The lengthy lens is a lens having function of correcting surface offset and curve of an image surface  each in the optical deflector and curve of an image surface such as a length cylinder lens or a lengthy toroidal lens or the like. The lengthy toroidal lens may also include a barrel-shaped toroidal surface as a concave lens surface with a curvature radius thereof in a direction corresponding to auxiliary scanning being smaller in accordance with its separation from the optical axis in a direction corresponding to the main scanning.
The second image-formation system may comprise a constant-velocity optical-scanning image-forming mirror and a lengthy toroidal lens as a lengthy lens provided on the side of the scanned surface other than the configuration by combining two types of lens as described above.
A lateral magnification xcex2 in a direction corresponding to the auxiliary scanning in a composite system of the optical system between the light source for a multi-beam and the scanned surface satisfies the following expression:
2 less than xcex2xe2x89xa68.5xe2x80x83xe2x80x83. . . (1)
and a plurality of light spots optically scan scanning lines adjacent to each other.
The lateral magnification xcex2 is made larger than 2, and by employing a xe2x80x9cscaling-up typexe2x80x9d of composite system, a lengthy lens included in the second image-formation system can effectively be separated from the scanned surface.
Also, when the lateral magnification xcex2 in the composite system is larger than 8.5 times, in order to realize a pitch of scanning lines of 84.7 xcexcm corresponding to the minimum dot density of 300 dpi required to the optical scanner, a space between light emitting sections in the light source for a multi-beam becomes not more than 10 xcexcm, and xe2x80x9cthermal crosstalk (a phenomenon that light emission in other light emitting section is affected by heating in one light emitting section)xe2x80x9d between LD light emitting sections or LED light emitting sections rapidly increases, so that it is difficult to control discretely blinking of each of the light emitting sections.
The light source for a multi-beam has also two LD light emitting sections, and the two light emitting sections can be provided at symmetric positions with respect to an optical axis of a collimate lens. In this case, as a light source for a multi-beam, one having a space between the two LD light emitting sections of 14 xcexcm is used, and a lateral magnification xcex2 in a direction corresponding to the auxiliary scanning in the composite system of an optical system between the light source for a multi-beam and the scanned surface can be made 4.536 times.
Further, by satisfying the conditions, the lengthy lens included in the second image-formation system can effectively be separated from the scanned surface, and in order to realize a pitch between scanning lines of 84.7 xcexcm corresponding to the minimum dot density of 300 dpi required to the optical scanner, a space between light emitting sections in the light source for a multi-beam can be maintained by not less than 10 xcexcm with which thermal crosstalk does not occur.
In the present invention, a diameter of a light spot or a pitch between scanning lines on the scanned surface are decided mainly by means of an image forming magnification in xe2x80x9can optical system in the side of the light source sidexe2x80x9d and an image-forming magnification in the second image-formation system according to a coupling lens and the first image formation system.
The image-forming magnification of the optical system in the side of the light source is decided by means of a magnification of the coupling lens and an image-forming magnification of the first image-formation system, however, a light flux coupled by the coupling lens is weak in converging performance or in diverging performance even in both cases where the light flux becomes a light flux to be converged and where it becomes a light flux to be diverged, so that a value of an image-forming magnification of the optical system in the side of the light source becomes substantially close to a ratio between a focal length of a coupling lens and that of the first image-formation system.
When the value becomes lower than a lower limit of the condition (1), a focal length in the first image-formation system for realizing a magnification required for an optical system in the side of the light source becomes smaller, and the first image-formation system approaches the optical deflector too close, which causes an obstacle for a layout of the optical arrangement. Especially, when the first image-formation system comprises xe2x80x9ca piece of lensxe2x80x9d, and the second image-formation system includes a constant-velocity optical-scanning image-forming mirror, sometimes there may occur a case where a light flux reflected on the constant-velocity optical-scanning image-forming mirror is truncated by the first image-formation system.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.